Case report: potential error in the estimation of antimicrobic concentrations by the microbiological assay method.

An 83-year-old man was admitted for gastrointestinal bleeding that required a segmental colectomy for correction. The patient was initially treated with gentamicin, clindamycin, and cephapirin for sepsis. Antibiotic coverage was changed to amikacin, clindamycin, and penicillin G for sepsis and anaerobic cellulitis. When renal function began to deteriorate, serum amikacin levels were determined by a radioenzymatic and bioassay method. The serum concentration of amikacin in the presence of penicillin G and clindamycin was overestimated when a multiantimicrobic-resistant Escherichia coli was used as the indicator organism in the microbiological assay.